1. Field of the Invention
The invention relates to an automatic adjustment device for internal shoe brakes, comprising a fixed housing disposed interiorly of a rotating brake drum, and a slidably guided, non-rotatable, axially (with respect to itself) displaceable pressure element disposed in the housing and operated mechanically or by a pressurized fluid. The pressure element acts on the brake shoe, via an adjustment bushing mounted in the pressure element in the housing, and via an adjusting bolt which is screw-connected to the adjustment bushing and is non-rotatably connected to the brake shoe.
2. Description of the Prior Art
In an automatic adjustment device of this general type described in German Pat. No. 2,631,704, the spring comprises a sheet having a rectangular cross section and is deformed to have a corrugated shape. This corrugated sheet spring is intended to press the ring (having the slot) against the adjustment bushing and thereby to hold the sawtoothed faces of the unidirectional clutch means together in engagement.
Although this well-known automatic adjustment device has proven to be the best available, it has been found that the corrugated sheet spring is subject to huge variations in service life under the frequently changing loads. This nonuniformity of service life is attributable to the manufacturing technique, namely it is attributable to wear phenomena on the stamping tooling. As is well known to one skilled in the art, when the sheet spring is stamped a punch burr or ridge is generated on the spring, on the exit side of the tool, which burr or ridge is more marked when the state of wear of the stamping tool is more advanced. The presence of this burr or ridge increases the chance of crack formation in the sheet spring, leading sooner or later to failure of the spring under load. Accordingly, attempts have been made to remove the burr or ridge, by special treatment of the stamped sheet spring. The treatment used is known as "Trowalizing". It is carried out in a grinding apparatus having a working container in the form of a helical path. The mixture of corrugated sheet springs and grinding bodies (namely small, shaped stones) is passed in a circular (i.e. helical) path from the bottom to the top level of the working container. However, an hour of such treatment is not sufficient to completely remove the burrs and ridges which may be present on the corrugated shape of the sheet spring. Moreover, it has been found that even after this relatively costly treatment the service life of the corrugated sheet springs still has wide variations. Accordingly,it is frequently necessary to replace broken corrugated sheet springs, which is a time-consuming and expensive procedure.